Unicast Datagram Routing

When registered on a foreign network, the mobile node uses the following rules to choose a default
router:

If the mobile node is registered and uses a foreign agent
care-of address, the process is straight forward. The mobile node chooses
its default router from among the router addresses that are advertised in
the ICMP router advertisement portion of that agent advertisement. The mobile
node can also consider the IP source address of the agent advertisement as
another possible choice for the IP address of a default router.

The mobile node might be registered directly with the home
agent by using a co-located care-of address. Then the mobile node chooses
its default router from among those routers that are advertised in any ICMP
router advertisement message that it receives. The network prefix of the chosen
default router must match the network prefix of the care-of address of the
mobile node that is externally obtained. The address might match the IP source
address of the agent advertisement under the network prefix. Then the mobile
node can also consider that IP source address as another possible choice for
the IP address of a default router.

If the mobile node is registered, a foreign agent that supports
reverse tunnels routes unicast datagrams from the mobile node to the home
agent through the reverse tunnel. If the mobile node is registered with a
foreign agent that provides reverse tunnel support, the mobile node must use
that foreign agent as its default router.